Respiratory morbidity, particularly chronic lung disease (CLD), remains a major cause of long-term morbidity and mortality for preterm infants. Although surfactant replacement has decreased acute respiratory morbidity and mortality, it has not reduced the incidence of CLD. A number of other approaches, including antenatal thyrotropin releasing hormone in conjunction with corticosteroids, postnatal steroid administration, as well as administration of Vitamin E, diuretics, and bronchodilators, have not resulted in clinically important decreases in CLD. Infants with the most severe CLD go on to develop findings suggestive of pulmonary hypertension with cor pulmonale. There is preliminary evidence in the preterm infant with severe chronic lung disease that low-dose inhaled nitric oxide may significantly attenuate the disease and decrease mortality. We propose a multi-center, controlled and blinded trial to investigate the hypothesis that low-dose inhaled nitric oxide administered to preterm infants between 500 and 1250 grams birth weight who continue to require mechanical ventilation at 10 days of age will increase survival without CLD at 36 weeks post menstrual age. Demonstrating an increase by 25% from 50% to 62% survival without CLD requires 726 randomized infants to have 80% power to detect this difference while controlling for "" of 0.05 and allowing for one interim analysis at one-third of outcome data available. Secondary outcomes are duration of ventilation, oxygen requirement and duration of hospitalization. We expect, in addition, that there will be improvement in infant respiratory status (ventilatory support, airway resistance and compliance) associated with inhaled nitric oxide treatment. Indicators of inflammation and oxidant stress will be assessed by measurements of specific cytokines and protein modifications in tracheal aspirate and plasma samples, respectively. We also will evaluate safety of this therapy by assessing toxicity as measured by clinical bleeding, including intraventricular hemorrhage as well as the incidence of other morbidities of the preterm infant (necrotizing enterocolitis, retinopathy of prematurity and infection) and assess neurodevelopmental outcome through two years of age. In summary, this clinical trial will assess the efficacy and safety of inhaled nitric oxide for amelioration of a major disease of premature infants.